Derivative Instruments - Non-Hedge Derivative Gains (Losses) in the Consolidated Statements of Operations (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012
Non-Hedge Derivative Gains (Losses) [Line Items]
Receipts from (payments on) settlement of non-hedge derivative instruments	$ (9,896)	$ (2,257)
Non-hedge derivative losses	(15,546)	(10,463)
Swaps
Non-Hedge Derivative Gains (Losses) [Line Items]
Change in fair value of non-hedge derivatives	(19,364)	(3,985)
Receipts from (payments on) settlement of non-hedge derivative instruments	4,525	2,701
Enhanced Swap [Member]
Non-Hedge Derivative Gains (Losses) [Line Items]
Change in fair value of non-hedge derivatives	2,031	0
Costless Collars
Non-Hedge Derivative Gains (Losses) [Line Items]
Change in fair value of non-hedge derivatives	(8,285)	(9,108)
Receipts from (payments on) settlement of non-hedge derivative instruments	5,731	33
Basis Protection Swaps
Non-Hedge Derivative Gains (Losses) [Line Items]
Change in fair value of non-hedge derivatives	176	373
Receipts from (payments on) settlement of non-hedge derivative instruments	$ (360)	$ (477)